Why empty Content-Type in nginx, php-fpm bundle?

Question

PHP

Good afternoon!
Faced a problem with getting HTTP headers.
I am using nginx and php-fpm. The task was to get an additional header and check it.
Used the alternative getallheaders function:

if (!function_exists('getallheaders'))
{
    function getallheaders()
    {
        $headers = '';
        foreach ($_SERVER as $name => $value)
        {
            if (substr($name, 0, 5) == 'HTTP_')
            {
                $headers[str_replace(' ', '-', ucwords(strtolower(str_replace('_', ' ', substr($name, 5)))))] = $value;
            }
        }
        return $headers;
    }
}

Variables with the HTTP_ key from the $_SERVER variable are written to it.
In the Nginx config, I specified:

location ~ \.php$ {
        include /etc/nginx/fastcgi_params;
        fastcgi_pass    127.0.0.1:9000;
        try_files  $uri =404;

        fastcgi_index   index.php;
        fastcgi_param   SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_param   CONTENT_TYPE    $content_type;
        fastcgi_param   CONTENT_LENGTH  $content_length;

        fastcgi_param   HTTP_HOST               $host;
        fastcgi_param   HTTP_CONTENT_TYPE       $content_type;
        fastcgi_param   HTTP_CONTENT_HMAC       $http_content_hmac;

        access_log  /var/log/nginx/postdata.log  main_post;
}

I can't figure out why the $_SERVER[CONTENT_TYPE] variable is passing an empty value?
The question is also of interest: how to specify an additional parameter in the config, in my example it is Content-HMAC. For output in the logs, the variable is specified "http_content_hmac"
Thank you in advance for your help!